In recent years, to lighten the steering force that has to be applied to a steering wheel and to provide a comfortable steering feel, electric power steering systems have been widely used in automotive vehicles. An electric power steering apparatus produces an assisting torque corresponding to a steering torque with an electric motor, applies this assisting torque to an output shaft or rack shaft of a steering mechanism, and steers steering road wheels with this output shaft or rack shaft.
Constructions for applying the assisting torque include those of a type wherein the assisting torque produced by the electric motor is transmitted to the output shaft or rack shaft by way of a belt type transmission mechanism. When this kind of belt type transmission mechanism is employed, the freedom of disposition of the electric motor is high. An example of a known electric power steering apparatus of this kind is the “ELCTRIC POWER STEERING APPARATUS” disclosed in Japanese Patent Publication No. HEI-7-94226.
This power steering apparatus has a construction wherein an output shaft is joined to an input shaft by a torsion bar and a steering torque applied to a steering wheel is transmitted to steering road wheels through the input shaft, the torsion bar, the output shaft and knuckles.
An angle of relative twist between the input shaft and the output shaft connected by the torsion bar is detected electrically with a steering torque sensor to detect the steering torque. A belt is passed around a small pulley mounted on the shaft of an electric motor and a large pulley mounted on the output shaft, and an assisting torque produced by the electric motor in correspondence with the steering torque is applied to the output shaft via the small pulley, the belt and the large pulley.
In this electric power steering apparatus of related art, the tensile force acting on the belt fluctuates with fluctuations in the assisting torque, and the bending load acting on the motor shaft of the electric motor from the small pulley also fluctuates. The motor shaft is a shaft extending outward from a motor case, and has the same bending rigidity as a “cantilever” or “overhang”. When the motor shaft is subjected to a large bending load and bends, because the belt leans to one side on the small pulley, the torque transmission efficiency tends to fall. Consequently, when the fluctuation of the assisting torque is large, irregularities occur in the assisting torque applied from the electric motor to the output shaft, and this is undesirable from the point of view of improving steering feel.
Also, in this related art power steering apparatus, by the amount through which the torsion bar twists in correspondence with the steering torque, a relative angular displacement arises between the input shaft and the output shaft. Consequently, the operation timing of the steering road wheels lags slightly behind the steering timing of the steering wheel. Furthermore, because the relative twist angle between the input shaft and the output shaft connected by the torsion bar is detected by a steering torque sensor, the detection timing also lags slightly, and the timing at which the electric motor produces the assisting torque lags.
Normally, the rotor and motor shaft of the electric motor have their own inherent inertia. Because the belt has to be flexible, generally its rigidity is small compared to metal transmission members such as steel gears, and it has an inherent spring constant. That is, the motor shaft of an electric motor having inertia is connected to an output shaft having a relatively large mass by a belt having an inherent spring constant. Consequently, the transmission timing of the assisting torque produced by the electric motor may lag slightly by the time it reaches the output shaft.
When the transmission timing lags like this, i.e. when a time lag occurs, it has a subtle influence on the steering feel.
Accordingly, in an electric power steering apparatus which apply an assisting torque produced by an electric motor to a steering mechanism by way of a belt type transmission mechanism, there has been a need for steering feel to be improved.